The Scharfenberg coupler[1] (German: Scharfenbergkupplung or Schaku) is a commonly used type of fully automatic railway coupling.

Designed in 1903 by Karl Scharfenberg in Königsberg, Germany (today Kaliningrad, Russia), it has gradually spread from transit trains to regular passenger service trains, although outside Europe its use is generally restricted to mass transit systems. The Schaku is superior in many ways to the AAR (Janney/Knuckle) coupler because it makes the electrical and also the pneumatic connections and disconnections automatic. However, there is no standard for the placement of these electro-pneumatic connections. Some rail companies have them placed on the sides while others have them placed above the mechanical portion of the Schaku. The main disadvantage to the Scharfenberg coupler is its low maximum tonnage, which makes it unsuitable for freight operations.

The coupler face has a protruding cone and a matching cup. Inside the cone there is a rigid metal hoop connected to a revolving, spring-loaded metal disk with a notch on the opposite side. When ready to couple the spring turns the disk so the hoop is extended from the cone, as the cars meet the hoop enters the cup on the other coupler, stopping against the disk. The hoops are then pressed back into their own coupler, causing the disks to rotate until the notches align with the hoops. After the hoops have entered the notches the disks spring back into the hoop extended position, locking the coupling. In the coupled position forces on the hoops and disk will balance out, which means that the Scharfenberg is not dependent on heavy latches to stay locked as many other couplers do.

Small air cylinders, acting on the rotating heads of the coupler, ensure the Schaku engagement, making it unnecessary to use shock to get a good coupling. Joining portions of a passenger train can be done at very low speed (less than 2 mph or 3.2 km/h in the final approach), so that the passengers are not jostled about. Rail equipment manufacturers such as Bombardier offer the Schaku as an option on their mass transit systems and their passenger cars and locomotives. In North America all the trains of the Montreal Metro are equipped with it, as are new light rail systems in Denver, Baltimore and New Jersey. It is also used on light rail vehicles in Portland, Minneapolis, some Australian multiple unit trains, the Vancouver Skytrain, and the Scarborough RT in Toronto. It also equips all the dedicated rolling stock used for the shuttle services in the Channel Tunnel. In Singapore, all trains (except APMs) uses Scharfenberg couplers.